I was just going to mention that the use of L-methylfolate could be another approach to the management of post-influenza hemostatic effects, given that L-methylfolate and other reduced folates appear to be active as cofactors for tetrahydrobiopterin (BH4)-dependent enzymes. It's something that researchers could investigate and that one would want to discuss with one's doctor. I think it might primarily be useful, conceivably, after the acute illness has abated, in part because BH4 is a cofactor for inducible nitric oxide synthase (iNOS) enzymes and not just eNOS. I think the higher dosage range of L-methylfolate might be useful under certain circumstances, such as in the "aftermath" of a viral illness and the oxidative stress that is associated with viral illnesses. Oxidative stress would tend to increase the rate of degradation of BH4 and to potentially increase the neopterin/biopterin ratio, thereby potentially leading to further, functional inhibition of either the formation of BH4 or of BH4-dependent enzymes (eNOS, tyrosine hydroxylase, and tryptophan hydroxylase). For example, Mittermayer et al. (2005) [Mittermayer et al., 2005: (http://ajpheart.physiology.org/cgi/reprint/289/4/H1752.pdf)(http://www.ncbi.nlm.nih.gov/pubmed/15964928?dopt=Abstract)] found that the intra-arterial administration of BH4 counteracted the detrimental effects of intravenous endotoxin on eNOS activity, basically (or it might have acted as an antioxidant, without enhancing eNOS activity). There's a large amount of research on BH4 in humans, now, and most of it shows that BH4 does increase eNOS-dependent blood flow responses (http://scholar.google.com/scholar?q=tetrahydrobiopterin+oral+OR+orally+OR+peroral+OR+supplement&hl=en&as_sdt=2001&as_sdtp=on). It's being tested in trials in the treatment of peripheral arterial disease, and maybe it'll be covered by insurance then. It's been approved to treat phenylketonuria, but I checked the out-of-pocket cost of it. It's prohibitively expensive. There's research showing it can pretty dramatically decrease blood pressure in people who are hypertensive. If only they'd started testing it 30 years ago, in 1981 or so, when some of the first research showing its biological effects was published. In the context of Alzheimer's disease, some researchers discussed the potential for an increase in BH4 availability to limit the uncoupling of eNOS activity (and nNOS and iNOS activities) to NO formation (or to uncouple the formation of citrulline, from arginine, from the formation of NO) and thereby decrease peroxynitrite formation [Foxton et al., 2007: (http://www.ncbi.nlm.nih.gov/pubmed/17191137)].
From the standpoint of the effects of methylfolate or BH4 on the brain, I think that L-methylfolate could initially increase, for example, iNOS activity in astrocytes or microglia or even increase nNOS activity excessively and produce effects on mood or cognitive functioning that would initially but not "ultimately" be counterproductive. For example, the administration of L-arginine via microdialysis into the caudal ventrolateral medulla (CVLM) augmented the pressor response to muscle contractions, but the arginine produced the opposite effect when administered into the rostral ventrolateral medulla (RVLM) [Freda et al., 1999: (http://www.ncbi.nlm.nih.gov/pubmed/10320724)]. Those effects could be explained in terms of differences in the anatomical "localization" of nitrergic inputs to glutamatergic neurons that provide excitatory inputs to the preganglionic sympathetic neurons in the RVLM, but most of these articles don't provide many details on the anatomy [Zanzinger et al., 1998: (http://ajpregu.physiology.org/cgi/reprint/275/1/R33.pdf)(http://www.ncbi.nlm.nih.gov/pubmed/9688957?dopt=Abstract)]. Another article found a pressor effect of arginine in the CVLM [Rampin and Giuliano, 2000: (http://www.ncbi.nlm.nih.gov/pubmed/10899272)] (I just glanced over part of that and thought that was on cardiovascular reflexes only). But the general idea is that nNOS-derived NO tends to be excitatory and to bind directly to NMDA receptors or to otherwise increase glutamatergic transmission, such as by amplifying the excitatory effects of NMDA receptor activation. But that's not really the way BH4 behaves in models of hypoxia or ischemia. There's some research showing that it worsens the damage, but a lot of it shows neutral or beneficial effects. Those parts of the medulla are mainly involved in cardiovascular reflexes, but the same type of concept applies to the nitrergic regulation of the firing rates of noradrenergic neurons in the locus ceruleus or of dopaminergic neurons, etc. BH4 or methylfolate could enhance nNOS activity and, by that mechanism and its function as a cofactor of tyrosine hydroxylase, exert an excitatory influence on dopaminergic and noradrenergic transmission (and serotonergic transmission), but there could be an initial enhancement of nNOS (and eNOS and iNOS) activity/activities and then a subsequent "normalization" of the nitrergic transmission. The increases in NO release, induced by BH4 or methylfolate, could, relatively rapidly, lead to a kind of new, equilibrium state and restore the normal extent to which nNOS-derived (and iNOS-derived) NO produces autoinhibition of nNOS and iNOS activities. For example, these authors [Galijasevic et al., 2003: (http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=299800)(http://www.ncbi.nlm.nih.gov/pubmed/14657339)] found that the presence of myeloperoxidase, a heme-containing enzyme present in neutrophils and other cell types, could function as a nitric oxide scavenger but paradoxically disinhibit iNOS activity by relieving the constitutive, autoinhibitory influence that iNOS-derived NO exerts on iNOS activity and that is thought to be largely responsible for the maintenance of iNOS activity at between 20 and 30 percent of its maximal rate (Galijasevic et al., 2003). The autoinhibition phenomena is a real phenomenon and isn't just a theoretical mechanism. I actually observed that type of phenomenon (in my opinion) when I took significant dosages of methylcobalamin during infectious mono, and it was mainly apparent when I increased the dosage (these were reasonable but sort of large dosages, as in 10 mg/d, even though there are 15 mg dosage forms of methylcobalamin now) and took it on an empty stomach. It always took 2 hours+ to be absorbed on an empty stomach (apparently because it's all bound to B12 binding proteins or forms some sorts of insoluble complexes?), and it behaved, initially, like a nitric oxide donor but then rapidly ceased to produce that type of effect (the migrating myoelectric complexes that drive gastric motility in the fasted state occur every 1.5 to 2 hours, incidentally, and "reset" upon the ingestion of small amounts of food, meaning it would take about 2 hours for the small amount of food to begin entering the small intestine, assuming it was part of a molecule too large to be transported into the small intestine in solution, by solvent drag or whatever, between the cells). It was possible to tell these things because of all the upregulation of iNOS activity, in response to the viral illness, etc. The point is that nitrergic effects, in response to something like methylfolate, seem to exhibit a kind of tachyphylaxis, and that's consistent with the transience of the inhibitory effects of high dosages of nitrate-derived NO on platelet function. There's an initial inhibition of platelet activation and platelet functions, but there's rapid adaptation/tolerance to the effect. But there still is a nitrergic effect of higher dosages of methylfolate, in the longer term, that's less pronounced than the effects that occur in the short term, in my opinion. In this article on the use of BH4 in the treatment of depression [Curtius et al., 1983: (http://www.ncbi.nlm.nih.gov/pubmed/6131342)], there was a delay of a couple of days until the maximal mood-elevating effect emerged, and the dosage regimes used in the articles on the use of BH4 itself, in treating depression, are bizarre. The authors (Curtius et al., 1983) used a dose that was probably excessively-high (1 g once and then a 100 mg/d maintenance dosage) and that could have compromised the efficacy of the BH4, in my view, given the potentially counterproductive effects that excesses of NO can have on glutamatergic and monoaminergic transmission, etc., in relation to mood or cognitive functioning. But the authors also were using tyrosine at a high dosage, and tyrosine has the potential to worsen depression, in my opinion, particularly at the high dosages that can allow for the autoinhibition of tyrosine hydroxylase activity by tyrosine itself to become significant. But the point is that the 100 mg/d dosage (used in other case reports, also) is similar to the highest dosage (90 mg/d) of methylfolate that's been used in small trials, in the treatment of depression. And the timecourse with which the changes in mood (or, by extension, in something like blood flow, etc.) emerged are consistent with some sort of adaptation to a change in nitrergic transmission. I generally haven't found that Rx L-methylfolate produces any more benefits at dosages above ~50 mg/day, but this hasn't always been the case. I just think that one has to work with one's doctor and try to individualize the dosage in a way that's appropriate for oneself, as an individual. In other words, there may be a significant need to individualize the dosage, and an awareness of some of the mechanisms by which BH4 acts, in the brain, may be useful in...the individualization of the dosage, etc.
Showing posts with label Tetrahydrobiopterin (BH4). Show all posts
Showing posts with label Tetrahydrobiopterin (BH4). Show all posts
Saturday, January 9, 2010
Tuesday, May 5, 2009
Evidence That Reduced Folates Can Serve as Cofactors for Tetrahydrobiopterin (BH4)-Dependent Enzymes
This is a really great article [Smith et al., 1986: (http://www.pubmedcentral.nih.gov/picrender.fcgi?artid=2418601&blobtype=pdf)(http://www.ncbi.nlm.nih.gov/pubmed/3540926)], and the authors discuss a lot of in vivo and in vitro research suggesting that reduced folates (i.e. levoleucovorin or L-methylfolate) can substitute (or their "metabolites," meaning tetrahydrofolate or other reduced folates whose concentrations are increased, intracellularly, in response to exogenous L-leucovorin or L-methylfolate) for tetrahydrobiopterin (BH4) as cofactors for tyrosine hydroxylase and other BH4-dependent enzymes. This is one of the articles that provides evidence of this [Lucock et al., 1995: (http://www.ncbi.nlm.nih.gov/pubmed/7566370)], and the authors of both articles discuss the fact that reduced folates are actually less neurotoxic than folic acid in people with dihydropteridine reductase deficiency (a genetic disorder that causes hypofunctionality in that enzyme, which is one of the BH4 biosynthetic enzymes). McCarty (2007) [McCarty, 2007: (http://www.ncbi.nlm.nih.gov/pubmed/17293058)] discussed evidence that methylfolate can serve as a substitute for BH4 ["pinch-hit[ting]" (McCarty, 2007, p. 14)] as a cofactor for endothelial nitric oxide synthase (eNOS).
The articles cited by Smith et al. (1986) also show, uniformly, as far as I can tell, that folic acid has no cofactor activity for tyrosine hydroxylase. That may not be all that relevant to the use of folic acid in brain disorders, because folic acid doesn't enter the brain nearly as readily as reduced folates do (supposedly, *only* reduced folates enter the brain, but this may not be true). But the authors of some of those articles suggest that folic acid could compete with reduced folates for binding to the transporters for reduced folates and thereby crowd out reduced folates from the brain.
The capacity of reduced folates to serve as BH4-mimetics is relevant to research on the uses of reduced folates in people with depression or in whatever other context. Their supposed capacity to serve as BH4 substitutes is different from their supposed capacity to promote BH4 recycling. I've discussed that research in past postings, and there's recent research that casts serious doubt on that "promotion of BH4-recycling," by reduced folates, as a mechanism. But if reduced folates can partially substitute for BH4, this would provide a rationale for the kinds of higher doses of reduced folates that have been used in the treatment of depression [Guaraldi et al., 1993; Di Palma et al., 1994, cited here: (http://hardcorephysiologyfun.blogspot.com/2009/01/thirty-year-old-articles-showing-folate.html)]. Those doses are similar to the maintenance dose of 100 mg/d of oral BH4 that has been used to treat depression [see references here: (http://hardcorephysiologyfun.blogspot.com/2009/03/d-phenylalanine-l-phenylalanine-l.html); Curtius et al., 1982: (http://www.springerlink.com/content/g1447716574g2616/)]. Obviously, anyone would want to discuss this with one's doctor before taking high doses of reduced folates, because a person with vitamin B12 deficiency could be seriously harmed by that approach. But in people who are not deficient in B12, Smith et al. (1986) basically say that reduced folates are less toxic than folic acid. Another important implication of "BH4-mimesis" by reduced folates is that reduced folates could conceivably exert feedback inhibition of GTP cyclohydrolase I, given that both BH4 and BH2 (and, by extension, methylfolate or levoleucovorin, as supposed BH4 analogs with cofactor activity for BH4-dependent enzymes) exert feedback inhibition of GTP cyclohydrolase I activity by binding to GTP cyclohydrolase I feedback regulatory protein (http://hardcorephysiologyfun.blogspot.com/2009/03/d-phenylalanine-l-phenylalanine-l.html). That could be an important mechanism by which BH4 or reduced folates could exert their supposed antidepressant effects, in my opinion, because, in people with BH4-responsive genetic disorders, that feedback inhibition causes reductions in the tissue and plasma concentrations of oxidized biopterins [biopterin, neopterin, different dihydrobiopterins (there's quinonoid dihydrobiopterin and 7,8-dihydrobiopterin, etc.)]. Oxidized biopterins can inhibit folate dependent enzymes and can also inhibit BH4-dependent enzymes, in some cases, by complex mechanisms. They basically disrupt pterin metabolism in a multitude of ways. But it seems as if researchers are reluctant to research those higher dosage ranges (50-90 mg/d), and that reluctance may be unfounded. Obviously, these are just my opinions. The authors that tested 90 mg/d didn't think the dose was toxic and said as much, but, obviously, that's just their opinion, also, and they didn't get any experimental data to support that assertion of safety. That's the type of thing that one would obviously want to not do in the absence of a doctor's supervision. BH4 itself is available by prescription in the U.S. (http://hardcorephysiologyfun.blogspot.com/2009/03/d-phenylalanine-l-phenylalanine-l.html), but it seems as if it might not be covered by insurance in some instances (for some applications). Also, everyone keeps repeating, in the research, that BH4 crosses the blood-brain barrier poorly (in comparison to sepiapterin, etc.), but who knows if that's even true. Researchers could compare BH4 to high (i.e. equimolar) doses of L-leucovorin or L-methylfolate and measure plasma biopterin and neopterin levels (the neopterin to biopterin ratio has tended to be elevated in people with depression, perhaps as a result of inflammation, etc.). Another possibility is that the feedback inhibition of GTP cyclohydrolase I activity by BH4 or L-methylfolate or L-leucovorin would "spare" guanosine triphosphate (GTP) and make more available to eNOS, etc. A lot of researchers have suggested that some of the effects of reduced folates on the brain could be due to increases in GTP or total guanosine nucleotide levels, as a result of L-methylfolate- or L-leucovorin-induced increases in de novo purine formation (i.e. in the brain or cerebral vascular endothelial cells, etc.). But the "feedback sparing" effect could conceivably occur, also. That would be interesting, because the formation of a lot of oxidized biopterins (such as neopterin) could be viewed as constituting a "wasting" and sequestration of guanosine nucleotides, which tend to be in short supply to begin with.
The articles cited by Smith et al. (1986) also show, uniformly, as far as I can tell, that folic acid has no cofactor activity for tyrosine hydroxylase. That may not be all that relevant to the use of folic acid in brain disorders, because folic acid doesn't enter the brain nearly as readily as reduced folates do (supposedly, *only* reduced folates enter the brain, but this may not be true). But the authors of some of those articles suggest that folic acid could compete with reduced folates for binding to the transporters for reduced folates and thereby crowd out reduced folates from the brain.
The capacity of reduced folates to serve as BH4-mimetics is relevant to research on the uses of reduced folates in people with depression or in whatever other context. Their supposed capacity to serve as BH4 substitutes is different from their supposed capacity to promote BH4 recycling. I've discussed that research in past postings, and there's recent research that casts serious doubt on that "promotion of BH4-recycling," by reduced folates, as a mechanism. But if reduced folates can partially substitute for BH4, this would provide a rationale for the kinds of higher doses of reduced folates that have been used in the treatment of depression [Guaraldi et al., 1993; Di Palma et al., 1994, cited here: (http://hardcorephysiologyfun.blogspot.com/2009/01/thirty-year-old-articles-showing-folate.html)]. Those doses are similar to the maintenance dose of 100 mg/d of oral BH4 that has been used to treat depression [see references here: (http://hardcorephysiologyfun.blogspot.com/2009/03/d-phenylalanine-l-phenylalanine-l.html); Curtius et al., 1982: (http://www.springerlink.com/content/g1447716574g2616/)]. Obviously, anyone would want to discuss this with one's doctor before taking high doses of reduced folates, because a person with vitamin B12 deficiency could be seriously harmed by that approach. But in people who are not deficient in B12, Smith et al. (1986) basically say that reduced folates are less toxic than folic acid. Another important implication of "BH4-mimesis" by reduced folates is that reduced folates could conceivably exert feedback inhibition of GTP cyclohydrolase I, given that both BH4 and BH2 (and, by extension, methylfolate or levoleucovorin, as supposed BH4 analogs with cofactor activity for BH4-dependent enzymes) exert feedback inhibition of GTP cyclohydrolase I activity by binding to GTP cyclohydrolase I feedback regulatory protein (http://hardcorephysiologyfun.blogspot.com/2009/03/d-phenylalanine-l-phenylalanine-l.html). That could be an important mechanism by which BH4 or reduced folates could exert their supposed antidepressant effects, in my opinion, because, in people with BH4-responsive genetic disorders, that feedback inhibition causes reductions in the tissue and plasma concentrations of oxidized biopterins [biopterin, neopterin, different dihydrobiopterins (there's quinonoid dihydrobiopterin and 7,8-dihydrobiopterin, etc.)]. Oxidized biopterins can inhibit folate dependent enzymes and can also inhibit BH4-dependent enzymes, in some cases, by complex mechanisms. They basically disrupt pterin metabolism in a multitude of ways. But it seems as if researchers are reluctant to research those higher dosage ranges (50-90 mg/d), and that reluctance may be unfounded. Obviously, these are just my opinions. The authors that tested 90 mg/d didn't think the dose was toxic and said as much, but, obviously, that's just their opinion, also, and they didn't get any experimental data to support that assertion of safety. That's the type of thing that one would obviously want to not do in the absence of a doctor's supervision. BH4 itself is available by prescription in the U.S. (http://hardcorephysiologyfun.blogspot.com/2009/03/d-phenylalanine-l-phenylalanine-l.html), but it seems as if it might not be covered by insurance in some instances (for some applications). Also, everyone keeps repeating, in the research, that BH4 crosses the blood-brain barrier poorly (in comparison to sepiapterin, etc.), but who knows if that's even true. Researchers could compare BH4 to high (i.e. equimolar) doses of L-leucovorin or L-methylfolate and measure plasma biopterin and neopterin levels (the neopterin to biopterin ratio has tended to be elevated in people with depression, perhaps as a result of inflammation, etc.). Another possibility is that the feedback inhibition of GTP cyclohydrolase I activity by BH4 or L-methylfolate or L-leucovorin would "spare" guanosine triphosphate (GTP) and make more available to eNOS, etc. A lot of researchers have suggested that some of the effects of reduced folates on the brain could be due to increases in GTP or total guanosine nucleotide levels, as a result of L-methylfolate- or L-leucovorin-induced increases in de novo purine formation (i.e. in the brain or cerebral vascular endothelial cells, etc.). But the "feedback sparing" effect could conceivably occur, also. That would be interesting, because the formation of a lot of oxidized biopterins (such as neopterin) could be viewed as constituting a "wasting" and sequestration of guanosine nucleotides, which tend to be in short supply to begin with.
Friday, December 26, 2008
No, Wait...(Note on Tetrahydrobiopterin)
No, I guess they're saying, in those two articles that I cited in my previous posting, that tetrahydrobiopterin (BH4) can be reduced by either/both methylenetetrahydrofolate reductase and dihydrofolate reductase. I can't bring myself to read through them really carefully right now. There's lots of research showing that methylfolate or folate can influence the cellular redox state and the ratio of oxidized to reduced glutathione, and that could influence BH4 regeneration more indirectly than the actions of those folate-derived-cofactor-dependent enzymes may.
I'm not saying there isn't an effect of reduced folates on BH4 regeneration, but it's not clear to me that those effects would occur because BH4 would be more efficiently regenerated, merely because of the presence of an excess of reduced folates or merely by virtue of the capacity, in vitro, of DHF and methylenetetrahydrofolate reductases to reduce BH4. Vitamin C, for example, *can* reduce BH4, too, but vitamin C isn't likely, in my opinion, to produce effects that resemble those of methylfolate. The effects of methylfolate on endothelial nitric oxide synthase are really strong, and I'll try to find, at some point, this one article that shows those effects. Endothelial nitric oxide synthase (and not just tyrosine and tryptophan hydroxylases) also requires BH4 as a cofactor, and the BH4 regeneration/substitution concept has been suggested, by researchers, as a mechanism that would account for the vasodilatory effects of reduced folates. But I think that many of the effects of reduced folates can be explained in terms of the effects of folate-derived cofactors acting as substrates or allosteric activators/inhibitors of folate-cofactor-dependent enzymes.
I'm not saying there isn't an effect of reduced folates on BH4 regeneration, but it's not clear to me that those effects would occur because BH4 would be more efficiently regenerated, merely because of the presence of an excess of reduced folates or merely by virtue of the capacity, in vitro, of DHF and methylenetetrahydrofolate reductases to reduce BH4. Vitamin C, for example, *can* reduce BH4, too, but vitamin C isn't likely, in my opinion, to produce effects that resemble those of methylfolate. The effects of methylfolate on endothelial nitric oxide synthase are really strong, and I'll try to find, at some point, this one article that shows those effects. Endothelial nitric oxide synthase (and not just tyrosine and tryptophan hydroxylases) also requires BH4 as a cofactor, and the BH4 regeneration/substitution concept has been suggested, by researchers, as a mechanism that would account for the vasodilatory effects of reduced folates. But I think that many of the effects of reduced folates can be explained in terms of the effects of folate-derived cofactors acting as substrates or allosteric activators/inhibitors of folate-cofactor-dependent enzymes.
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